1. Field of the Invention
The present invention is related to the control of steam turbines and, more particularly, to a method and system for controlling the inlet valves in different operating modes.
2. Description of the Related Art
The majority of electricity generated in the U.S. is generated by steam turbines that receive steam from a boiler heated by fossil fuel or a nuclear reactor. The amount of electricity produced by the turbine is controlled by inlet valves which determine how much of the steam is supplied to the steam turbine. Typically, there are six tg eight inlet valves which are operated in one of two modes. In a single valve or unison mode, all of the valves are opened at the same rate. This mode is used most often in starting the turbine so that the rotor is evenly heated by the entering steam.
A partially opened valve introduces throttling loss of energy from the steam and if all of the valves are partially opened, there is considerably more loss than if only one or two of the valves are partially open. As a result, a sequential mode of operation is used during the majoritY of the time that the turbine is operated. In the sequential operation mode, first a group of three or four valves are opened at the same rate until they are fully opened, or nearly so. Then, if additional steam flow is demanded, another one or two valves are opened to control the operation of the turbine, nnd when they are nearly fully opened then another one or two valves are opened, etc. until the turbine is controlled by the last one or two valves or is operating at maximum capacity.
The lift versus flow characteristics of inlet valves are non-linear. In recognition of this fact, conventional control systems store the lift versus flow characteristic of the valves for converting flow demand to valve lift or position. However, only a single flow-to-lift characteristic is stored and adjustments to the characteristic are equally applicable to both unison and sequential operation modes. Systems using such conventional systems are disclosed in U.S. Pat. Nos. 4,270,055; 4,368,520; 4,418,285; 4,512,185; and 4,554,788, among others, all issued to the assignee of the present invention and incorporated herein by reference.